1. Technical Field
The present invention relates to a high-frequency power amplifier and, particularly to a wide-band high-frequency power amplifier that is suitable for transmission of a mobile communication device including a mobile phone.
2. Background Art
In recent years, for mobile communication devices such as mobile phones, there is a strong demand for multiband in which a plurality of frequency bands are used because of reservation of a communication capacity and global roaming. Currently mainstream communication systems include a second generation system in which GSM (Global System for Mobile Communications) is adopted and a third generation system in which CDMA (Code Division Multiple Access) is adopted. Band I ranging from 1920 MHz to 1980 MHz, Band II ranging from 1850 MHz to 1910 MHz, Band III ranging from 1710 MHz to 1785 MHz, Band IV ranging from 1710 MHz to 1755 MHz, Band V ranging from 824 MHz to 849 MHz, Band VI ranging from 830 MHz to 840 MHz, Band VIII ranging from 880 MHz to 915 MHz, Band IX ranging from 1749.9 MHz to 1784.9 MHz, and Band XI ranging from 1427.9 MHz to 1452.9 MHz exist as the frequency band used in transmission of the communication systems. A combination of the communication system and a communication frequency differs in various regions where the mobile phone is used.
In such multiband mobile phones, in addition to downsizing of a battery, it is necessary that efficiency (power saving) of transmission power amplifier in which a power consumption ratio is high be improved in order to achieve downsizing, weight reduction, and prolonged talk. A GaAs high-frequency transistor excellent in a high-frequency characteristic and power conversion efficiency is mainly used as the transmission power amplifier for the mobile phone. The GaAs high-frequency transistor is roughly classified into a field effect transistor (hereinafter referred to as an FET) and a heterojunction bipolar transistor (hereinafter referred to as an HBT).
In CDMA systems such as W-CDMA, a technique of controlling the high-frequency power output from an antenna of the mobile terminal according to a distance to a base station or an ambient environment is used such that the high-frequency power reaching the base station is substantially equalized. Generally, the antenna output increases in the case that the mobile terminal is far from the base station, and the antenna output decreases in the case that the mobile terminal is close to the base station. The antenna output is performed by controlling the output of a PA (Power Amplifier) module (hereinafter referred to as a “high-frequency power amplifier”). Frequently the mobile terminal is used while the antenna output is suppressed to a relatively low level, and it is necessary to achieve the high efficiency of the high-frequency power amplifier in a low output condition in order to reduce the power consumption. However, the high-frequency power amplifier is designed such that power efficiency is maximized during high output. Therefore, the power efficiency is degraded in the output power below the high output.
A high-frequency power amplifier that switches an output route used according to the required output power is proposed as a technology of improving the efficiency during medium output and low output (see U.S. Patent Publication No. 2007/0222523). FIG. 9 is a block diagram of high-frequency power amplifier 29 disclosed in U.S. Patent Publication No. 2007/0222523.
Conventional high-frequency power amplifier 29 will be described below with reference to FIG. 9. Note that, in the following description, the same element is designated by the same numeral. High-frequency power amplifier 29 includes input terminal 1, output terminal 2, bias/control circuit 3, high output route 10, medium output route 100, low output route 110, and output matching circuit 13.
An operation of high output route 10 will now be described. A high-frequency power signal input from input terminal 1 is input to high-output amplifier 11 through input matching circuit 12, and amplified. The high-frequency power signal amplified by high-output amplifier 11 is output from output terminal 2 through output matching circuit 13.
Next, the operation of medium output route 100 will be described. The high-frequency power signal input from input terminal 1 is input to medium-output amplifier 111 through input matching circuit 112. The high-frequency power signal amplified by medium-output amplifier 111 is output from output terminal 2 through output matching circuit 113, switch element 114, and output matching circuit 13.
Next, the operation of low output route 110 will be described. The high-frequency power signal input from input terminal 1 is input to low-output amplifier 211 through input matching circuit 212. The high-frequency power signal amplified by the low-output amplifier 211 is output from output terminal 2 through output matching circuit 213, switch element 214, output matching circuit 113, switch element 114, and output matching circuit 13 in turn. Which one of three output routes 10, 100, and 110 amplifies the high-frequency power signal input from input terminal 1 is determined by bias/control circuit 3, which performs control of a bias current to amplifiers 11, 111, and 211 and control of switch elements 114 and 214.
Thus, the conventional high-frequency power amplifier 29 switches the output route used according to the required output power.